1. Field of the Invention
The present invention relates a projection zoom lens of a 5-group configuration and 3-group movement mounted on a projection type display device or the like and a projection type display device having the projection zoom lens, and more particularly, to a projection zoom lens and a projection type display device suitable for projecting on the large screen in a movie theater or the like.
2. Description of Related Art
A projector device (projection type display device) having a relatively long back focal length using a light valve such as a liquid crystal display and a DMD display has come into wide use (see JP-A-8-201690).
Recently, in movie theaters, a projector that is suitable for a large screen and is capable of projecting a higher-precision picture has been used as such a projector device.
A reflection type liquid crystal display element or a DMD 3-sheet type is used for the projector device provided for such a use, and a longer back focal length and telecentricity as satisfactory as the other projectors are necessary.
Generally, a value obtained by dividing a projection distance by a screen width is referred to as a throw ratio. A screen size and a distance from a screen to a projection room, that is, a projection distance are various according to movie theaters. Accordingly, to project a picture with a proper size for each movie theater, a lens corresponding to each proper throw ratio is necessary, but it is not a good method to actually prepare all lenses corresponding to proper throw ratios in consideration of cost. Thus, this can be addressed by providing a width that can correspond to the throw ratio using a zoom lens.
However, in most of the known lenses for a projector, Fno is changed during zooming.
Generally, Fno on a TELE side is slower than that on a WIDE side. Accordingly, in such a zoom lens, a picture of a movie theater with a high throw ratio gets dark even in movie theaters having the same screen size.
As a zoom ratio of a lens gets higher, a wide usability of the lens gets higher. However, in the known lens for a projector, as a zoom ratio gets higher, a variable of Fno gets larger. Accordingly, the lens is not suitable for a movie theater.
To address that the lens is not suitable for a movie theater since Fno is varied according to change of the zoom ratio, there has been proposed a lens described in JP-A-2002-122782.
Since the lens described in JP-A-2002-122782 is a varifocal lens, telecentricity is not kept constantly during power-varying and an operation is complicated when power-varying is necessary after installation.